1. Field of the Invention
The present invention relates to a system for removing particles from exhaust produced by internal combustion engines.
2. Description of the Prior Art
Exhaust produced by internal combustion engines, particularly diesel engines, includes many exhaust particles. The principal constituents of which are carbon and other components which are soluble in organic solvents (These components will be referred to as SOF). Therefore, in order to clean the exhaust for preventing the emission of exhaust particles into the atmosphere, a filter is provided within an exhaust passage to trap particles.
Such an exhaust particle removing system is described in part in S.A.E. paper 88006 and Japanese Patent First publications (Tokkai Sho.) Nos. 57-212318, 59-150918 and 60-150414.
In internal combustion engines, as described in the aforementioned publications, a filter for trapping exhaust particles is disposed within an exhaust passage of the engine. The exhaust passage branches upstream of the filter for establishing a bypass passage within which a muffler or silencer is disposed. At the branching point between the bypass passage and the exhaust passage, a directional control valve is provided. In addition, a burner is also disposed upstream of the filter. The burner causes exhaust particles, which were trapped by the filter to be deposited thereon, to reborn to preventing the pores of the filter from being blocked.
While the vehicle is in motion, the bypass passage is usually closed by means of the directional control valve, and the exhaust passes through the filter so that the exhaust particles are trapped by the filter to clean the exhaust.
An exhaust-pressure sensor monitors whether or not the volume of the exhaust particles trapped by the filter is greater than a preselected value. When it is detected that the trapped volume becomes greater than the preselected value, the exhaust particles trapped in the filter are burned by the burner to reclaim the filter. In this way, the exhaust pressure is so controlled as to prevent it from increasing excessively, so that reduced the output and increased fuel consumption of the internal combustion engine is prevented.
Since a great amount of exhaust particles synchronously begin to burn during reclaiming the filter, it is noted that if the preselected value is too high, cracking occurs in the filter and/or the filter is damaged by melting thereof. On the other hand, when the aforementioned preselected value is too low, reclaiming must be frequently performed, so that fuel consumption is increased and degradation of the burner is accelerated. Therefore, the preselected value is carefully selected to balance these points.
However, in such conventional exhaust particle removing systems, particularly in the case of motor coaches, trucks and so forth, there are the following disadvantages.
Since the volume of the filter mounted on a motor coach is very large, a great difference in temperature distribution occurs within the filter during reclaiming, so that reclaiming is not performed evenly. In the worst case, burnout occurs due to heat distortion in the filter.
With conventional filters, there is another disadvantage in that gaseous SOF constituents in the exhaust particles can not be trapped, so that they are emitted into the atmosphere.